diff --git a/synapse/storage/prepare_database.py b/synapse/storage/prepare_database.py
index b364719312..bd740e1e45 100644
--- a/synapse/storage/prepare_database.py
+++ b/synapse/storage/prepare_database.py
@@ -25,7 +25,7 @@ logger = logging.getLogger(__name__)
 
 # Remember to update this number every time a change is made to database
 # schema files, so the users will be informed on server restarts.
-SCHEMA_VERSION = 51
+SCHEMA_VERSION = 52
 
 dir_path = os.path.abspath(os.path.dirname(__file__))
 
diff --git a/synapse/storage/schema/delta/52/add_event_to_state_group_index.sql b/synapse/storage/schema/delta/52/add_event_to_state_group_index.sql
new file mode 100644
index 0000000000..91e03d13e1
--- /dev/null
+++ b/synapse/storage/schema/delta/52/add_event_to_state_group_index.sql
@@ -0,0 +1,19 @@
+/* Copyright 2018 New Vector Ltd
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+-- This is needed to efficiently check for unreferenced state groups during
+-- purge. Added events_to_state_group(state_group) index
+INSERT into background_updates (update_name, progress_json)
+    VALUES ('event_to_state_groups_sg_index', '{}');
diff --git a/synapse/storage/state.py b/synapse/storage/state.py
index b88c7dc091..3f08f447a3 100644
--- a/synapse/storage/state.py
+++ b/synapse/storage/state.py
@@ -1114,6 +1114,7 @@ class StateStore(StateGroupWorkerStore, BackgroundUpdateStore):
     STATE_GROUP_DEDUPLICATION_UPDATE_NAME = "state_group_state_deduplication"
     STATE_GROUP_INDEX_UPDATE_NAME = "state_group_state_type_index"
     CURRENT_STATE_INDEX_UPDATE_NAME = "current_state_members_idx"
+    EVENT_STATE_GROUP_INDEX_UPDATE_NAME = "event_to_state_groups_sg_index"
 
     def __init__(self, db_conn, hs):
         super(StateStore, self).__init__(db_conn, hs)
@@ -1132,6 +1133,12 @@ class StateStore(StateGroupWorkerStore, BackgroundUpdateStore):
             columns=["state_key"],
             where_clause="type='m.room.member'",
         )
+        self.register_background_index_update(
+            self.EVENT_STATE_GROUP_INDEX_UPDATE_NAME,
+            index_name="event_to_state_groups_sg_index",
+            table="event_to_state_groups",
+            columns=["state_group"],
+        )
 
     def _store_event_state_mappings_txn(self, txn, events_and_contexts):
         state_groups = {}